1. Field of the Invention
The present invention relates to a network system using at least a multiplex line, and more particularly to a network system in which a plurality of terminals are serially connected using at least a multiplex line and a network system using a DQDB protocol as a transmission protocol.
2. Related Background Art
In recent years, as a system for transmitting and receiving data among a plurality of computers has been developed, there has occurred a desire for establishing a large number of communication channels in the system. In order to increase the number of the communication channels, there have been presently proposed a method for increasing the multiplex magnitude of time division multiplexing by increasing the transmission capacity of a transmission line and a method for increasing the number of wavelength multiplex channels by converting respective data into optical signals of different wavelengths and transmitting these optical signals to an optical fiber.
In the method of multiplexing optical signals at plural different wavelengths in an optical fiber, transmission data can be assigned to each wavelength and the following communication can be attained.
At present, there exist Ethernet, FDDI (Fiber Distributed Data Interface), DQDB (Distributed Queue of the Dual Bus), FFOL (FDDI Follow On Lan) and so forth, as communication protocols for local area networks. Those protocols can be used as optical communication protocols for the respective wavelengths, and thus respective terminals can freely perform communications based on those respective communication protocols only by using a single optical fiber to connect those terminals. Further, since the transmission speed of each communication protocol is standardized, the transmission capacity needed in the network system can be flexibly coped with by using the same communication protocol for the respective optical communications of the plural different wavelengths.
Further, Japanese Patent Laid-Open Application Nos. 7-99501 and 7-312586 disclose art according to which there are arranged a main signal line having a plurality of multiplex channels and a control signal line having channels for transmitting signals in mutually-opposite directions, a request slot is transmitted through one of the channels in the control signal line, and the acquisition of a channel management table transmitted through the other channel in the control signal line is executed on the basis of the count value of the request slot. Further, there is also disclosed art for multiplexing the main signal line and the control signal line and art for performing assignment of a portion of channels in the control signal line by using the channel assign method. Those references, however, do not disclose art for providing a plurality of channels for at least one of the mutually-opposite directional channels in the control signal line and multiplexing those plural channels.